1. Field of the Invention
The present invention relates to a current-excitation light-emitting element. Further, the present invention relates to a light-emitting device and an electronic device including the light-emitting element.
2. Description of the Related Art
In recent years, research and development have been actively conducted on light-emitting elements using electroluminescence. In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained.
Light-emitting elements using electroluminescence are broadly classified according to whether the light-emitting substance is an organic compound or an inorganic compound.
In the case where an organic compound is used for the light-emitting substance, by applying voltage to a light-emitting element, electrons and holes are injected from a pair of electrodes into a layer containing a light-emitting substance, so that current flows therethrough. Then, by recombination of these carriers (electrons and holes), the light-emitting substance forms an excited state, and emits light when the excited state returns to a ground state. Because of such a mechanism, such a light-emitting element is called a current-excitation light-emitting element.
As described above, the current-excitation light-emitting element using an organic compound as a light-emitting substance can be manufactured to be thin and lightweight and can be driven at low voltage. Therefore, the current-excitation light-emitting element is thought to be suitable for a next-generation flat panel display element. In addition, response speed being extremely fast is also one of the characteristics, and it is also an advantage that high image quality can be realized by applying the current-excitation light-emitting element to a display.
Further, since such a light-emitting element can be formed in a film form, planar light emission in a large area can be easily obtained. This implies that a “surface light source” can be formed easily which is difficult to obtain by a point light source typified by an incandescent lamp or an LED, or by a line light source typified by a fluorescent lamp. Therefore, utility value as a surface light source that can be applied to lighting or the like is also high.
Although such a light-emitting element can provide various emission colors depending on the kind of light-emitting substances, development of a light-emitting element which provides white light is important in consideration of application to a flat panel display or lighting. This is not only because a full-color display can be realized with combination of a white light-emitting element and a color filter, but also because white light is an emission color with the highest demand for lighting.
When a plurality of kinds of organic compounds showing different emission colors is combined, light emission of desired color, light emission having a broad spectrum, or white light emission can be obtained. However, light-emitting efficiency and lifetime of the light-emitting element described above are great problems, which also apply to a white light-emitting element.
In order to solve these problems, light-emitting efficiency and lifetime of the white light-emitting element are improved by using a specific substance in, for example, Patent Document 1. However, the measures are yet inadequate in consideration for practical application.
In the case of a light-emitting element using a plurality of kinds of organic compounds showing different emission colors, adjustment in order to obtain light emission of desired color is difficult in some cases due to influence of energy transfer or the like.